Laser interferometers o the preamble-basing type are particularly suitable for high-resolution distance of displacement measurement and a so vibration or oscillation measurement. The measurement can be performed in non-contacting manner and optically without any mechanical influencing of the object to b measured. Coherent light, which is preferably produced by laser light source, is split into the object light and reference light provided for interaction with the object to be measured. The object light is separately guided from the reference light in a measurement arm to the object to be measured, interacting therewith e.g. by reflection and is then superimposed with the reference light guided in a reference arm This leads to interference patterns, which are highly sensitive to the measurement arm length and the time variation thereof.
In the measurement of vibrating objects to be measured so-called heterodyne interferometers have been adopted, which inter alia allow a correct sign-characterization of the oscillations or vibrations. They are in particular characterized by at least one frequency shift device for producing a frequency shift between the object light or beam and the reference light or beam. This frequency shift can in particular be produced through the use of an opto-acoustic modulator, particularly a Bragg cell.
Heterodyne interferometers can be used for measurements on objects to be measured with optically smooth surfaces (e.g. mirrors, retroreflectors, etc.) and on such objects having optically rough surfaces. If the reflectivity of the object to be measured is in the lower range in which the interferometer is able to measure, then the precision or resolution of the measurement within a given measurement integration time is dependent on the reflected back object light power.
In the case of an optimum interferometer design, it is known to determine the lowest resolution limit attainable with conventional means through so-called shot noise of photons. Hereinafter this limit is also referred to as the quantum noise of photons or the quantum noise limit (QNL).
Scientific experiments, e.g. in the light beam communication field or astronomical interferometry have indicated that it is possible to perform measurements close to the quantum noise limit if on the detection side use is made of a so-called balance detecting device for converting reference light and object light into electrical signals (G. L. Abbas, V. W. S. Chan and T. K. Yee “Local-oscillator excess-noise suppression for homodyne and heterodyne detection”. Optics Letters, vol. 8, no. 8 (1983), pp 419 to 421 or H. van de Stad “Heterodyne Detection at a Wavelength of 3.39 μm for Astronomical Purposes”, Astron. & Astrophys. 36 (1974), pp 341 to 348). The detecting device proposed in the first publication for balanced detection has two identically constructed detectors with in each case a photoelectric cell and a suitable circuit for converting the photoelectric cell current caused by the incidence of light into a detector output voltage. For forming an evaluatable difference signal the detector output voltages are subtracted from one another, so that signal components based on steady light components in both detectors can be eliminated from the amplified subtraction signal.
The desire to measure as close as possible to the quantum noise limit is confronted in the case of heterodyne interferometers by the requirement for a measurement with a maximum heterodyne frequency. For a number of reasons, high heterodyne frequencies are desirable. Firstly, the interference or background noise can be suppressed in a broader band manner the higher the heterodyne frequency. Secondly, the permitted measurement object speed or the band width of the vibrations to be measured can be made higher the higher the heterodyne frequency is chosen. Finally, the most acousto-optical modulators can be given a smaller size and electric power consumption the higher the heterodyne frequency is chosen. However, the heterodyne frequency cannot be chosen in a arbitrarily high manner, because on the detection side the thermal noise of load resistors, where the measurement currents of optoelectronic transducers drop, increased in proportion to the heterodyne frequency level. Therefore in the case of higher heterodyne frequencies for obtaining resolutions in the quantum noise limit range, the reference light power should be made particularly high, so that the photon noise exceeds the thermal noise and becomes detectable.
Bearing this in mind it is possible to in principle perform a measurement with resolution in the quantum noise limit range. However, the structures used in research are too complicated and expensive or their environmental demands are too high, so that the presently commercially available interferometers do not reach the quantum noise limit.
Preamble-basing interference devices can be used with particular advantage in the medical field for obtaining data on the hearing power, e.g. via direct vibration measurements on the tympanum or eardrum. Examples of such measurements by means of laser Doppler vibrometry are e.g. described in international patent application WO 97/04706 or in the publication by N. Stasche, H. J. Foth and K. Hörmann in HNO, 1993, 41, pp 1 to 6. The desire to measure as close as possible to the quantum noise limit here results from a naturally low reflectivity of the object to be measured (tympanum) and secondly through the wish to be able to measure extremely small oscillation or vibration amplitudes down to the picometer range, such as occur close to the hearing threshold in the medium frequency range. The problem is exacerbated by the fact that for practical applications, e.g. when treating patients on an outpatient basis in the clinic or through the ENT doctor, relatively short measurement times of typically less than one minute are sought.
The problem of the invention is to provide a preamble-basing device, which can be manufactured less expensively and which permits distance and/or vibration measurements with high resolution in the vicinity of the quantum noise limit. In particular, a highly sensitive, inexpensively manufacturable laser Doppler vibrometer is to be provided, which is in particular usable for the outpatient determination of data concerning the hearing power.